Semipermeable osmotic membrane and method of producing same

ABSTRACT

Semipermeable membranes prepared from a mixture of vinylpyrrolidone polymers and polyisocyantes. Process for preparing such membranes by casting the mixture from solution onto a smooth surface followed by curing under conditions of controlled humidity. Reaction of polyisocyanates with certain compounds having an active hydrogen atom provides a &#39;&#39;&#39;&#39;blocked&#39;&#39;&#39;&#39; polyisocyanate which does not react immediately with water at ambient temperatures thereby allowing greater processing control of membrane production.

United States Patent Riley et al.

[ 1 May 9,1972

[54] SEMIPERMEABLE OSMOTIC MEMBRANE AND METHOD OF PRODUCING SAME [72]Inventors: Robert L. Riley, La Jolla; Clifford R.

Lyons, San Diego, both of Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Interi- [22] Filed: Oct.3l,l969

[21] Appl.No.: 873,116

[52] U.S.Cl ..117/16l UA,117/124E,161/109, 161/112, 161/190, 210/500,260/2.5 AY, 260/25 [51] Int. Cl. ..C09d 3/72, C09d 5/20 [58] FieldofSearch ..117/124E, 161 UA, 161 UN, 117/98, 99; 264/41, 49; 161/109,112,190;260/2.5

AY, 2.5 AT; 210/500 [5 6] References Cited UNITED STATES PATENTS3,171,799 3/1965 Batchelder ..210/500 X 3,216,579 11/1965 Shelanski eta1. ..2l0/500 X 3,332,894 7/1967 Cantor et al.' ..264/41 X OTHERPUBLICATIONS Saline Water Conversion Report, 1968, US Dept. of theinterior, pages 123- 125.

Markle et al., pages 22- 25, 1964. Development of improvedMembranes...Dialysis Primary ExaminerWilliam D. Martin AssistantE.\'aminerBernard D. Pianalto Attorney-Ernest S. Cohen and GerstenSadowsky [5 7 ABSTRACT 6 Claims, N0 Drawings SEMIPERMEABLE OSMOTICMEMBRANE AND METHOD OF PRODUCING SAME This invention relates tosemipermeable membranes, and more particularly to such membranes whichare prepared from vinylpyrrolidone polymers and polyisocyanates, and toprocesses for making such semipermeable membranes.

It is known to employ semipermeable membranes to separate variouscomponents from various fluid mixtures. For example, semipermeablemembranes are employed to separate liquid components from a mixture ofgases, and to separate liquid solvents from a liquid solution of aliquid solvent and a dissolved solute. The development of membranereverse osmosis processes for the desalination of sea and brackish wateris one application of such fluid component separations in whichconsiderable development has taken place.

Osmosis is a concentration equalization process in which fluid istransported through a semipermeable membrane (which is permeable to saidfluid) from a region of greater concentration of said fluid to a regionof lesser concentration of that fluid. For example, if sea water isplaced on one side of a semipermeable membrane which is much morepermeable to water than to the ions of the salts dissolved in sea water,and pure water is placed on the other side of the membrane, then theconcentration equalization process of osmosis will result in a flow ofpure water through the membrane into the saltwater region where theconcentration of water is less.

In reverse osmosis, pressure in excess of the threshold pressure for aparticular fluid and its concentration differential across a membranepermeable to it (termed osmotic pressure), is applied to the region oflesser concentration so that the normal osmotic flow is reversed. Forexample, using a semipermeable membrane which is much more permeable towater than to the ions of the salts dissolved in sea water, pressure inexcess of the osmotic pressure may be applied to a region of sea wateron one side of such a membrane, resulting in the concentration of thesale ions in the sea water, and allowing the recovery of relatively purewater from the other side of the membrane.

The basic principles of reverse osmosis have been relatively wellunderstood for decades, but the development and construction of suitablesemipermeable membranes capable of withstanding substantial pressureswhile providing both high degree of component selectivity andflow-through rate has provided difficulties for the full development ofdesalination processes. The materials principally employed for theconstruction of prior art reverse osmosis membranes have generally beenorganic cellulose derivatives such as cellulose acetate, cellulosepropionate, cellulose acetate-butrate, and ethyl cellulose. Because ofits relatively high flux and good salt exclusion characteristics,cellulose acetate has probably been most extensively used and studied inreverse osmosis membrane applications. High quality, imperfection-freecellulose acetate membranes, however, are difficult to prepare. Byimperfections is meant openings in the semipermeable membrane throughwhich the feed mixture may pass without any rejection of substances towhich the membrane material is relatively less permeable.

In addition, semipermeable membranes such as cellulose acetate membranesoften have limited pH ranges of operation because of adverse hydrolysiseffects at pH values outside such limited ranges.

It is an object of the present invention to provide semipermeablemembranes having improved resistance to hydrolysis over a wide pH range.It is another object to provide reverse osmosis membranes which arereadily prepared in a substantially imperfection-free form, andprocesses of readily producing them. It is a further object to provide awide spectrum of semipermeable membranes in which membranecharacteristics such as component selectivity, fluid sorbtion, and fluidflow-through rate may be controlled and varied to fit the particularapplication.

Very generally, the instant invention provides semipermeable membranesprepared from a mixture of a vinylpyrrolidone polymer and apolyisocyanate, the mixture being cured with water under conditions ofcontrolled humidity. Also provided are processes for producing suchsemipermeable membranes. Particularly useful polyisocyanates areblocked" polyisocyanates which have decreased reactivity toward water atambient temperatures, but which provide reactive polyisocyanatefunctionality toward water at elevated temperatures.

vinylpyrrolidone is a vinyl monomer which may readily be polymerizedthrough the vinyl group to form essentially linear. water-solublevinylpyrrolidone polymers. Other vinyl monomers may, of course, bepolymerized with vinylpyrrolidone to provide vinylpyrrolidone polymerswith modified properties which are also useful herein. vinylpyrrolidonepolymers having a very broad range of molecular weights may besuccessfully employed in the various aspects of this invention. Themolecular weight of the vinylpyrrolidone polymer employed is oneparameter which may be varied to control the thickness and productioncharacteristics of the membranes of the instant invention. Polymers withlower molecular weight are less viscous in solution than highermolecular weight polymers of the same chemical constituency, and thisvariation in solution viscosity may be advantageously employed inthin-film production. Preferred are those vinylpyrrolidone polymerswhich have a molecular weight of at least about 5,000 and which containat least about 50 percent by weight polymerized vinylpyrrolidone.Particularly preferred are those polymers which have a molecular weightof at least about 10,000 and contain at least about percent by weightpolymerized vinylpyrrolidone. Polymers which have been found to beparticularly useful herein, and which have excellent film-formingproperties are homopolymers of vinylpyrrolidone. Preferred are thosehomopolymers of vinylpyrrolidone (polyvinylpyrrolidone) which have anaverage molecular weight in excess of about 100,000. The termvinylpyrrolidone as used herein includes mixtures of vinylpyrrolidonepolymers.

The vinylpyrrolidone polymers useful herein are preferably insubstantially anhydrous form prior to mixing with the polyisocyanatebecause it has been found that premature reaction of water with thepolyisocyanate may thereby be prevented.

Polyisocyanates useful herein are those compounds having at least dualisocyanate functionality and which are soluble in suitable organicsolvents such as chloroform, N, N dimethyl formamide or acetone. Suchpolyisocyanate compounds, which have found extensive use in urethanecoating and foam applications and which are described in thatliterature, are reactive toward compounds that have active hydrogenatoms, such as phenols, alcohols, amines and water. Examples of suchpolyisocyanate compounds useful herein are diisocyanates, such ashexamethylene diisocyanate, m-phenylene diisocyanate, 2,4-toluenediisocyanate, 2,6-toluene diisocyanate, methylenebis-(4-phenylisocyanate), dianisidine diisocyanate, and tolidinediisocyanate. Polymethylene polyphenylisocyanate is an example of apolyisocyanate useful herein which has more than dual isocyanatefunctionality. lsocyanatecapped prepolymers, such as those used in themanufacture of urethane foams, may also be advantageously employedherein. An example of such an isocyanate-capped prepolymer is thereaction product of excess toluene diisocyanate with 2-ethyl- 2-hydroxymethyl- 1 3-propane-diol:

The term polyisocyanate as used herein includes mixtures of differentpolyisocyanates.

lsocyanates react with certain compounds having active hydrogen atoms toform comppunds which are termed blocked isocyanates or splitters becausethe reaction is reversible at elevated temperatures. Blocked isocyanatesare described in the literature, for example by J. H. Saunders abd K. C.Frisch, Polyurethanes: Chemistry and Technology, Part I, V. XVI, and inHigh Polymers, Interscience Publisher, New York, 1962. Blockedpolyisocyanates are particularly useful herein. The reaction product ofa blocked polyisocyanate with water is considered herein to be includedwithin the scope of the reaction product of the polyisocyanate, fromwhich it may be produced. Phenol, guaiacol, and e-caprolactam areexamples of-compounds which may be used for blocking the hereinabovedescribed polyisocyanates to provide useful, blocked polyisocyanates.For example, e-caprolactam-blocked polymethylene .polyphenylisocyanatehas been successfully reacted with water in the presence ofpolyvinylpyrrolidone to produce the semipermeable membranes of theinstant invention.

dred times the difference between the feed and product waterconcentrations dividedby the feed water concentration, For

example, in sea water, the ions which make up the salt are the secondcomponent or components. If salt water having an initial percentage ofone weight percent sodium chloride is fed to a semipermeable membraneseparation means and the liquid which permeates through that separationmeans has a sodium chloride content of 0.05 weight percent, the membraneis said to exhibit a 95 percent rejection of sodium chloride. The otherparameter is a measure of the. rate of flow permeating the membrane andis termed the membrane constant. For purposes of this application, themembrane constant is measured as grams of output liquid per squarecentimeter of membrane surface, second of operation, and atmosphere netpressure (applied pressure minus osmotic pressure). Accordingly, themembrane constant is hereafter stated in terms of out? sou-still.

. competing in asense because it can be seen that generally asemipermeable membrane which allows a greater amount of fluid to passthrough it (either as a result of its inherent chemical and/or physicalstructure or as a result of minor imperfections therein) may not beasdiscriminating in rejecting the second component, i.e., salt, as willa similar membrane which allows a lesser amount of fluid to flow throughit at a given pressure. Of course, ideally a semipermeable membrane ismost desirable which has both a very high percent of rejection and avery high membrane constant. However, for many purposes, one of theseparameters may outweigh the other in specific importance so that,dependentupon the characteristic of feed mixture and upon the desiredcriteria of the product, a lower percent of rejection might be toleratedin order to obtain a higher product output from a membrane of givensurface area. For example, when.brackish water is used as the input feedmixture so that the concentration of dissolved solids is relatively low,a lower percent of rejection may be tolerated'in order to achieve afairly high output of drinking water having a dissolved solids contentno higher than a certain'desired level. semipermeable membranes of thisinventionmay be prepared having a wide range of combinations of percentof rejection and membrane constant, for example, through control of theamount and type of vinylpyrrolidone polymer and polyisocyanate materialused, and the fabrication procedure. In addition, membranecharacteristics may be varied by varying such parameters as the membranethickness.

Curing of a mixture of a vinylpyrrolidone polymer and a polyisocyanatewith water under controlled conditions ofhumidity provides an insoluble,hydrophilic material which, when placed in water at ambient temperaturewill absorb water and swell to a certain degree. In general, as therelative amount of polyisocyanate which is mixed with a vinylpyrrolidonepolymer increases, a series of hydrophilic products is provided whichhave a decreasing tendency to swell when immersed in water. it has beenfound that membranes which have a water-sorption of less than about 40percent, have improved ability to discriminate between feed-mixturecomponents. Thus, these membranes having this limited tendency to sorbwater have an improved semipermeable character with respect to membraneswith a greater tendency to sorb water. The term water-sorption is usedas a quantitive measure of the tendency of a membrane to swell whenimmersed in water. Water-sorption, as used herein, is defined astheweight percentage of water, based on the weight of the water-swollenmembrane, sorbed at equilibrium by a membrane when it is immersed inwater at 25 C. To produce the semipermeable membranes of this invention,a sufficient amount of polyisocyanate is mixed with a vinylpyrrolidonepolymer and the mixture cured in a film form with water under conditionsof controlled humidity to provide a membrane which has a watersorptionof less than about 40 percent. It is preferred that a sufficient amountof polyisocyanate be used to provide a membrane with a water-sorption offrom about 10 percent to about 20 percent.

Various theories might be advanced to explainthe formation and benefitsof the membranes of this invention. However, although it is not intendedthat this invention be bound by explanation, it is believed that themixture of vinylpyr rolidone polymer and polyisocyanate reactprincipally with water to form a polymeric biuret and by-product carbondioxide:

of vinylpyrrolidone polymer and polymerized polyisocyanate.

The relative insolubility of the vinylpyrrolidone polymers in suchintertwined polymer systems may result from factors such as hydrogenbonding between the vinylpyrrolidone polymer'and the polymerizedpolyisocyanate (coupled with their intertwined structure), or occasionalreaction of isocyanate groups with the relatively acidic hydrogen at thenumber 3 1 position of the polymerized vinylpyrrolidone. Blockedisocyanates are believed to react similarly with water at sufficientlyelevated temperatures to regenerate isocyanate functionality.

According to this theory, as the relative amount of polyisocyanate andits subsequent polymerization and crosslinking increases, thewater-sorption of the intertwined polymer system decreases. It is alsotheorized that the water-sorbed in films with a high water-sorption ofabout 40 percent or more is present in the form of interconnectedclusters which permit coupled flows of solvent and solute to occur. Assufficient polyisocyanate is utilized to provide membranes withwatersorptions decreasing from about 40 percent to about 20 percent, itis theorized that the water clustering diminishes, and that membraneshaving water-sorptions of less than about 20 percent have little coupledflow of solvent and solute and hence better solute exclusioncharacteristics. These theories are in no way limiting upon the scope ofthe invention, but

rather are offered to illustrate a possible explanation for itsadvantages.

In any event, varying the ratio of a particular polyisocyanate used witha vinylpyrrolidone polymer will provide variation in the water-sorptionof the resultant membrane. Generally, an increase in the relative amountof polyisocyanate will provide a decrease in water-sorption. In order toprovide membranes having the desired semipermeable characteristics,sufficient polyisocyanate should be used to provide a water-sorption ofless than about 40 percent, and preferably from about percent to aboutpercent.

The amount of polyisocyanate or blocked polyisocyante which is used toprovide a membrane with a water-sorption of less than about 40 percentwill vary depending upon the nature of both the vinylpyrrolidone polymerand the polyisocyanate or blocked polyisocyanate. For example, someimportant factors are the molecular weight of the vinylpyrrolidonepolymer, and the nature of monomers (if any), other thanvinylpyrrolidone incorporated into it, as well as the functionality (e.g., diisocyanate, triisocyanate, etc.) and other properties of thepolyisocyanate or blocked polyisocyanate. In addition, the condition ofhumidity as well as the temperature at which the reaction takes place,i.e., the cure temperature, may also affect the water-sorption of theresulting membrane.

Although the proportions of the particular materials which will producea membrane with the desired water-sorption may vary, the ratio of molesor isocyanate functionality of the polyisocyanate to moles ofvinylpyrrolidone in the vinylpyrrolidone polymer (termed herein theisocyanate equivalent) is usefully correlated with water-sorption. Sucha correlation is particularly useful when membranes are produced from ahomopolymer of vinylpyrrolidone and an aromatic polyisocyanate.

By proportioning the ratio of moles of isocyanate functionality of anaromatic polyisocyanate or blocked aromatic polyisocyanate to moles ofpolymerizedvinylpyrrolidone in the vinylpyrrolidone polymer so thatthere is an isocyanate equivalent of at least about 0.25, membraneshaving a watersorption of less than about 40 percent may usually beproduced. An isocyanate equivalent of between about 0.4 and 2.0 ispreferred. For example 1 mole of homopolymerized vinylpyrrolidone mixedwith 0.45 mole of methylene bis-(4 phenylisocyanate) may be cured at 150by air which has enough water vapor that it would have 5 percentrelative humidity at C., to provide a membrane with a water sorption of18 percent. Since there are 2 moles of isocyanate functionality per moleof methylene bis-(4 phenylisocyanate), the isocyanate equivalent in theabove example is 0.9.

The semipermeable membranes of this invention may vary in thickness fromless than about 1 micron to more than about One suitable method forfabricating these membranes is coating a suitable substrate with asolution of the polyisocyanate and vinylpyrrolidone polymer (known ascasting) followed by evaporation of the solvent from, and curing of thefilm under suitably controlled conditions of humidity. The substrateswhich may be employed include those having a smooth surface which willbe unaffected by the casting solution and which will either naturallyallow release of the semipen'neable membrane after solvent removal andcuring, or which can be suitably treated with a releasing agent toeffect such a release. Examples of suitable substrate materials includeglass, metals such as stainless steel, and plastics such as Teflon(polytetrafluoroethylene). When the substrate is a porous supportmembrane, it is of course not necessary that the membrane be releasedfrom its surface. The desired membrane thickness may readily be obtainedthrough control of the solution concentration and of the thickness ofthe later of solution applied to the substrate. Preferred methods ofcoating the substrate with a solution of the vinylpyrrolidone polymerand polyisocyanate are those, such as dipping and spraying, which willprovide an even coating and thereby result in the production of amembrane with uniform thickness.

Polyisocyanates are very reactive toward water, and if thepolyisocyanate reacts with excessive water prior to casting, themembranes will have a tendency to coagulate and form holes as thesolvent evaporates. In order to cast substantially imperfection-freemembranes from solution, therefore, it is 200 microns, and may beemployed in conventional reverse 7 generally exhibit a higher membraneconstant than a thicker membrane for a given percent of rejection. Thethinner membranes may require support during use, particularly atrelatively high operating pressures, for example by being disposedagainst a porous support membrane. Membranes having a thickness of lessthan about 50 microns are preferred. Particularly preferred are thosemembranes having a thickness of less than about 10 microns.

The semipermeable membranes of this invention are prepared by mixing avinylpyrrolidone polymer with a polyisocyanate and curing a film of themixture under conditions of controlled humidity. Although the finishedmembranes may be bent of folded, their shape becomes substantially fixedduring the'reaction, and therefore should be fabricated into a thin filmbefore this is prevented by the progress of the reaction.

advantageous to dissolve the vinylpyrrolidone polymer and thepolyisocyanate in a dry solvent prior to casting. The vinylpyrrolidonepolymer and polyisocyanate are proportioned in this casting solution sothat a membrane having the desired water-sorption will be produced.

When membranes are cast in this manner, the vinylpyrrolidone polymer andthe polyisocyanate may be dissolved in any suitable dry solvents ormixtures thereof which do not have a deleterious effect on thevinylpyrrolidone polymer, polyisocyanate or the curing of the membrane.It is noted that water has a deleterious effect upon the polyisocyanate,so that the suitable solvents used to dissolve either thevinylpyrrolidone polymer or the polyisocyanate should be substantiallyanhydrous. In the event that different solvents or mixtures thereof areused to prepare the solution of vinylpyrrolidone polymer andpolyisocyanate, these solvents should be selected so that the mixtureforms a homogeneous solution suitable for casting of a membrane.Examples of suitable solvents are substantially anhydrous N, N dimethylformamide, chloroform, and pyridine.

The concentration of the vinylpyrrolidone polymer-polyisocyanate mixturein the casting solution may be varied to fit the requirements of theparticular membrane casting operation.

For example, if the casting operation involves the application of aparticular thickness of the casting solution to a substrate (such as bya doctor blade) the thickness of the finished membrane may be controlledthrough variation of the concentration of the casting solution. If thesolution viscosity plays an important part in the casting operation,such as in spraying or dipping, the proper viscosity may be selectedthrough control of the molecular weight and concentration of thevinylpyrrolidone polymer and polyisocyanate in the casting solution.After the casting solution has been applied to the substrate, thesolvent is evaporated therefrom, and the resulting uncured membrane filmis cured under controlled conditions of humidity to providesemipermeable membranes. By controlled conditions of humidity is meantconditions of limited humidity at which the rate of cure of the membraneis lower than that at which the rate of production of the gaseousby-product (carbon dioxide) causes the membrane to blister or formimperfections. A useful methodof controlling the conditions of humidityis to cure the membrane in an atmosphere which contains limited amountsof water vapor. For example, a preferred method is to cure the membranein an atmosphere in which there is only enough water vapor present toprovide, at 1 atmosphere of pressure and 25 C., a relative humidity ofless than about 50 percent, and preferably between about 5 mosphere at agiven temperature to the maximum saturation vapor pressure of water inthat same atmosphere at the same blocked polyisocyanate, usually fromabout 140 C. to about 160 C. Through the use of blocked polyisocyanates,the

' deleterious effect of the progress of the cure upon the casting ofmembranes from solution may be completely eliminated.

temperature, Air and nitrogen .are useful atmospheres, For example,ultrathin and asymmetric membranes which are although the use of otheratmospheres is also contemplated. u stantially imperfection-free may bereadily prepared. In Also contemplated are other methods of controllinghumidity addition, the use of blocked polyisocyantes facilitates theconsuch as by curing the membranes with water vapor, or a mixtinuOuS F gP membranes Without having hh ture of water vapor and other gases, atreduced pressures so S- Memhrahesvwhh W Values of that the concentrationof water vapor is reduced below that at less than about 20 p hh P p f"I" which the rate of formation of carbon dioxide in the memamouhls ofblocked p y y i h" fahflcaledr brane will blister or cause imperfectionsduring curing. even m ultrathm form. and are submmlallyimperfection-free- Heat may be applied to facilitate the removal of s lvnt The following examples includedetailed descriptions of from the filmof casting solution, or to facilitate the cure of the semlpehheablemembranes and Pmcases P P S lhflh membrane, or both. The temperature atwhich the membrane which embody. various features of the invention. Itshould be is cured may affect the properties of the cured membrane.understood, however, that the following examples in no way Whilethepolyisocyanate is very sensitive to water so that final limit th pe fthe inventiOh which i d r l ly y th curirigmay be-effected for exampleat room temperature, it is elalms appearing at the end of thisspecification. preferred that the membrane be heated to an elevatedtemperature below the decompositiontemperature of the mem- EXAMPLE]brane. A preferred range of curing temperatures is between Anhydrouspolyvinylpyn-olidone pvp with an average about 35 and about molecularweight of 360,000 is dissolved in an anhydrous solr f l l l f aPhydmusFollmon of a l vent consisting of equal volumes of pyridine and N, N'-py s pq y 1s fmxed th polylsocyanate 801mm dimethyl formamide. Methylenebis-(4 phenyliso-cyanate) f a can"! solution comammg sufilclem amount of(MD!) is dissolved in an anhydrous solvent also consisting of to P amembrane havmg a P equal volumes of pyridine and N,N'-dimethyl formamideto l f than abmn Percemfl A mm of casttng form a series of solutionshaving different concentrations. The t on IS rapidly cast on a glassplate In a chamber Y "l polyvinylpyrrolidone solution is mixed with theseries of MDI 9 f of about 5 Percent about 50 pefcent lame solutions toprovide a series of 3 casting solutions each conhumidity at 25 C. and 1atmosphere of pressure is passed over mining 135 weight percent PVP, andvarying amounts of the surface of the film. After about 8 hours ofdrying, the films MDL These 3 casting Solutions are each immediatelypaston are heated ma hot air oven at 120 C. for 4 hours to facilitate cleanglass plates in a chamber in which air containing an complete solventrcmovfll mfembmne i amount of water vapor such that it would have'arelative humcmbrane has been cast m f or cxamp e on t 6 8 midity of 10percent at 25 C and 1 atmosphere of pressure, is a large,porous-walltube, it IS removed from the substrate passed over thesurface of the filmwoated p ates. After 2 a IS 2' i y thm lhi' hours ofdrying in this chamber, the films are heated in an "3 0 :55 t an 3 out 2may oven at 120 C. for 4 hours. The cured semipermeable memrcqum t {use9' 390m Support mem lane '8 Pres 40 branes are removed from the glassplates by soaking in water, Sim? opgmuo-n' u may be advantageous tocasta porous Supand are transparent strong and imperfection-free Thewaterc lt g; t; :jifgfigggf sorption of each membrane is measured bysoaking the memz s s g g gg z gsg from the slbstrate brane in distilledwater for 48 hours at room temperature, a g g of theemcgbranc are g yfacilitated blotting the membrane and then weighing it. The water isthen lmperfection free membranes of high quality are produced removedfrom l i 3 teglperaiure by the above-described process of casting andcuring memvaclwaover 21 5 mem hrane 7 F branes from solutions ofvinylpyrrolidone polymers and agam' e wa er content 0 t e F en SW0 Fpolyisocyanates V wafi ter at room temperature at eqlllllbllul: is thenobtained by r I di erence and the water-so tion is t en determined as aAn alternative process, -to circumvent the limitations of rp castingmembranes from solution which arise because of the welght'percem' basedon F of the water'swonen sensitivity of the polyisocyanate to waterwhich may be in the membrane Reverse osmosls expenmellm are Performed oncasting solution or in the atmosphere during casting, upon f h membraneusl'ng test separatmn devlce generally Contact with water, involves theuse a blqcked polyiso similar to that described rn U. S. Pat No. 3 1331332. A 0.8 cyanate' As mentioned hereinabove polyisocyanates may beweight percent aqueous sodium chloride solution is fed to the blocked byreacting certain types of active hydrogen com- Separauon devlfcgfdlacemto the fi g f membrahf ounds with the isoc anate rou 5. Such blocked olisoat a Pressure 0 atmospheres, an a flowlof this games do notreactywith i Z room temperatulsesflbut feed solution is maintained pastthe membrane to kecp'the efhave limited thermal stability so that theisocyanate groups feet of the boundary layer at a l J The membrane canbe regenerated at elevated temperatures so that the memstamgmd 2?perchehgof S llf l j Calcutta! 5 branesmay be cured. A casting solutionof a vinylpyrrolidone mam ram t F 0W rate t t e mam h an t e polymer anda blocked polyisocyanate may be stored at room salt concentration in theoutput fluid. The following Table l temperature, and cast whenconvenient, even in the presence p f h data for these membranes. Thelsocyanate of moisture. The solvent is then evaporated and the membraneequlvakhh heafhhg represents the h Present each of i cured b i i htemperature f h i l lid the casting solutions used to prepare themembranes, of moles polymer-blocked polyisocyanate film to at least thetemperaof isocyahate groups of the L to "10165 0f Pyrrohdohe ture rangeat which the polyisocyanate is regenerated from the 8 "P TABLE IMembrane constant Isocyassuming 0.2;. anate Thick- Watcrmembranethickness Salt Water Sodium chloride semipermeable momcquivnoss sorptiou(gJcmfl-scm-atm.) rejection pormeability permeability brane number alent(microns) (percent) (percent) (10" g./cm. -s0c.) (l0" cm. /s0c.

0. 44s 15 31 0.05x10- 35.8 27.00 1,400.00 0. 020 so 26 a. 00 10- 58.010. 00 200. 00 1.100 57 16 0. 20x10- 00.05 1.70 0.47

EXAMPLE n 3 6.7 (distilled water) 4.7 4 10.0 (NaOl-l) 4.0 5 11.0(NaOl-l) 3.1 6 12.4 (NaOl-l) 4.9

Under the pH conditions to which membranes O1 and 4-6 are exposed,cellulose acetate would be converted to cellulose in a few hours and becompletely destroyed for desalination purposes.

EXAMPLE V A sufficient amount of polyvinylpyrrolidone (PVP) andecaprolactam blocked polymethylene polyphenylisocyanate Membraneconstant Semipr-rmeable PMPI Waterassuming (1.2;; Salt Water Sodiumchloride. nn-mbrnne equivnsorption membrane thickness rejectionpermeability permeability number lent (percent) (g./em. -sec.-ntm.)(percent) (10- g./em.--sm-.) (111" "(-i1i.-/svv.)

0. 45 34 9. 21X10' 2. 4 25.1) 1355. (l 0. T0 28 3.13X10' 72. 3 8. 511-1. 1) 1.20 17 8.11Xl0 97.25 3.2 1.8

EXAMPLE Ill (Blocked PMPI) are dissolved in choroform to provide a cast-A series of casting solutions each containing percentpolyvinylpyrrolidone (PVP) and different amounts of polymethylenepolyphenylisocyanate which has been blocked by reaction withe-caprolactam (Blocked PMPl) is prepared using chloroform as thesolvent. These casting solutions are stable at room temperature, andcasting need not take place in an anhydrous environment in order toprepare imperfectionfree films. These casting solutions are each cast onclean glass plates in a chamber in which substantially dry air is passedover the surface of the film-coated plates. When the films have driedcompletely, a corner of each is tested to find that they remain solublein chloroform, indicating that reaction has not taken place. The filmsare then heated to 150 C. for 30 minutes in the presence of aircontaining a sufficient amount of water vapor to provide a relativehumidity of about 20 percent at 1 atmosphere to pressure and themembranes thus produced are removed from the glass plates by soaking inwater. They are then tested as in Example I.

The lsocyanate equivalent heading represents the ratio, present in eachof the casting solutions, of moles of e-caprolactam blocked isocyanategroups to moles of pyrrolidone groups. The cured membranes aretransparent, strong and imperfection-free. Table III presents this datafor these membranes.

ing solution containing only 0.50 percent by weight, based on the weightof the total solution, of PVP and Blocked PMPl. The ratio ofe-caprolactam blocked isocyanate groups to polymerized vinylpyrrolidonegroups is 0.28, sufficient to provide a membrane with a water-sorptionof about 38 percent. The PVP (which is identical to that used in ExampleI), the Blocked PMPI, and the chloroform are all substantiallyanhydrous, and the casting solution prepared from them is kept in a dryatmosphere. A clean, flat glass plate is immersed in this castingsolution for five minutes and then is withdrawn vertically into a dry,dust free chamber. The excess solution drips from the glass plate backinto the solution container, leaving a thin, uniform coating of thisvery dilute casting solution on the glass plate.

The chloroform solvent is allowed to evaporate and the ultrathinsemipermeable membrane is cured on the glass plate by heating to atemperature of 150 C. for 30 minutes in air containing a sufficientamount of water vapor to provide a relative humidity of about 20 percentat 25 C. and 1 atmosphere of pressure. A porous support membranecomposed of cellulose nitrate and cellulose acetate is then castdirectly on the ultrathin membrane while it is still adhering to theglass plate. The composite membrane, i.e., the combination of theultrathin semipermeable membrane and the porous support membrane, isreadily removed from the glass plate by soaking TABLE III Isocy-Membrane constant Semipermenble :mnte Wnterassuming 0.2;1. mern- Salt.Water Sodium chloride membrane number eqniv- Thickness sorption branethickness rejection permeability permeability nlent (microns) (percent)(g./em. -sec.-atm,) (percent) (1O" g./em -sec.) (lU- cmJ/sec.)

l). 3 80 37 1. 88x10 37. 5 51. ll 2, 150. ()(l l). 7 2!) 5. y4 10 (i4. 315, 3 2 15. ()0 1.2 115 22 8.11X10' 117.2 2.2 1.81) 1.6 65 17 (11BX10518. 8 1. 8 [1.63

EXAMPLE 1V in water. This composite membrane is tested by means of theTABLE IV pH of Storage Water (acid or base used) NaCl permeabilityMembrane No. (10'" cmlsec) 1.1 (HCl) 2 4.0 HCl test device used inExample I with the membrane side of the composite oriented toward thefeed mixture. The feed mixture consists of a 1.7 percent sodium chloridesolution at 24 C. applied at a pressure of 500 p.s.i. The resistance tosolution flow through the porous support membrane is small in comparisonwith that of the semipermeable membrane. The thickness of thesemipermeable membrane is estimated to be between 2 to 5 microns. Thesalt rejection is 60 percent and the membrane constant is 0.66 X 10g./cm. -sec.-atm.

The percent of salt rejection predicted for this membrane by assuming asolution-diffusion membrane mechanism, and based on the permeabilityratio of water and sodium chloride, was only about 20 percent.

EXAMPLE VI A porous support membrane (millipore VFWP) is sprayed on thesmall-pore side with a 1.4 percent, by weight, solution of polyacrylicacid (molecular weight 50,000) dissolved in equal volumes of water andethanol. The coated support membrane is then dried. The polyacrylic acidcoating is then coated with a 0.13 weight percent solution ofpolyvinylpyrrolidone (PVP-the same as that used in Example I) andpolymethylene polyphenylisocyanate (PMPI) having a relative amount ofPMPl sufficient to provide an isocyanate equivalent of 1.2. The coatingof the PVP-PMPI film takes place in a controlled humidity box containingair with relative humidity of percent at 25 C. The composite membrane isthen heated to 100 C. for minutes and tested by means of the devicedescribed in Example I using a 1 percent by weight sodium chloridesolution as a feed solution at an operating pressure of 800 p.s.i. Themembrane has a salt rejection of 96 percent and a membrane constant of0.17 X l0""g./cm. -sec.- atm.

The semipermeable membrane separation means which are described in thisapplication may be employed for treating a variety of feed mixtures,such as mixtures of two or more gases, mixtures of two or more liquids,for example water and organic liquids, liquids wherein solutes aredissolved, as for example sea water, fruit juices, whey, and acid minewaters. However, the operation thereof is sometimes hereinbeforedescribed with reference to sea water or saline water, one im portantpresent use for such semipermeable membranes. The term feed mixture asused in this application, includes mixtures of liquids regardless oftheir mutual solubility, mixtures of different gases, and solutionswherein mixture of a solid and a liquid results in the dissolution of asolid as a solute in the liquid, as well as combinations of theforegoing. Included within the scope of feed mixtures from whichseparations may be obtained by the instant invention are emulsions,suspensions, and biological media such as bacterial, viral and fungalcultures.

Imperfections and deleterious to semipermeable membrane performance,particularly at the high pressures desired for reverse osmosisapplications. Practice of the disclosed process readily producessemipermeable membranes which may be substantially imperfection-free.The resultant semipermeable membranes exhibit excellenthydrolysis-resistance which allows useful operational lifetimes for usein a pH range of from about 1.5 to about 12.5, thus providing a widescope of usefulness for practical semipermeable membrane technology. Anadditional advantage is that membranes of this invention may bespecifically tailored" to a particular need. Membranes having a widerange of properties, such as membrane constant, percent of rejection,etc., may readily be prepared for a particular engineering application.1

Various features of the invention are set forth in the claims whichfollow.

What is claimed is: v

l. A process for producing a semipermeable membrane comprising:

a. preparing a solution of a vinylpyrrolidone polymer and blockedpolyisocyanate,

b. coating a substrate with a film of said solution,

c. evaporating the solvent from said film to provide a membrane film,

d. heating said membrane film to a temperature at which said blockedpolyisocyanate is converted into a polyisocyanate, and

e. curing said membrane film under conditions of controlled humiditywherein the moisture concentration is insufficient to promote too rapida reaction of water with the polyvinylpyrrolidone and polyisocyanatewhich would cause blistering or imperfections in the membrane.

2. A process according to claim 1 wherein the ratio of blockedisocyanate groups to polymerized vinylpyrrolidone groups is greater thanabout 0.25. I

3. A process according to claim 1 wherein said curing is carried out inan atmosphere containing sufficient water vapor to provide saidatmosphere with a relative humidity of less than about 20 percent at25.20 C.

4. A process according to claim 3 wherein said vinylpyrrolidone polymeris a polyvinylpyrrolidone having an average molecular weight in excessof about 100,000, wherein said ratio is between about 0.4 and about 2.0,and wherein said temperature is from about C. to about C.

5. A process according to claim 4 wherein said blocked polyisocyanate isthe reaction product of a polyisocyanate with e-caprolactam.

6. A process according to claim 4 wherein said polyisocyanate is ablocked polymethylene polyphenylisocyanate.

2. A process according to claim 1 wherein the ratio of blockedisocyanate groups to polymerized vinylpyrrolidone groups is greater thanabout 0.25.
 3. A process according to claim 1 wherein said curing iscarried oUt in an atmosphere containing sufficient water vapor toprovide said atmosphere with a relative humidity of less than about 20percent at 25.20 C.
 4. A process according to claim 3 wherein saidvinylpyrrolidone polymer is a polyvinylpyrrolidone having an averagemolecular weight in excess of about 100,000, wherein said ratio isbetween about 0.4 and about 2.0, and wherein said temperature is fromabout 140* C. to about 175* C.
 5. A process according to claim 4 whereinsaid blocked polyisocyanate is the reaction product of a polyisocyanatewith epsilon -caprolactam.
 6. A process according to claim 4 whereinsaid polyisocyanate is a blocked polymethylene polyphenylisocyanate.